Sound demostrations

Sound wave are caused by vibrations and carry energy through a medium

• What is a sound wave?

Humans hear sounds waves in a limited frequency range

• From 20 vibrations per second (20 Hertz) to 20,000 vibrations per second (20,000 Hertz)

• Sounds below the range of human hearing is Infrasound.

• Sounds with a frequency above the range of human hearing is Ultrasound.

• Sound meter and oscilloscope (A device for viewing oscillations, as of electrical voltage or current, by a display on the screen of a cathode-ray tube.)

• Use Microphone and applets on computer.

Pitch – a measure of how high or low a

sound is and depends on frequency

• Penny Whistle• Homemade Penny whistle (need straws and scissors)

– Make and then cut to change pitch– Make and add a hole at the end

• Boom Wackers

• How does wavelength affect pitch?• The longer the wavelength the lower the pitch.

Mechanical waves need a medium

• Clock in a vacuum

Medium affects the speed of the wave• Oven racks (vibrations, pitch and frequency)• Can telephone

Speed of sound

• In Air (0 degrees C) = 331 m/s• In Air (100 degrees C) = 386 m/s• Oxygen = (0 degrees C) = 317 m/s• In Water = 1,490 m/s• Copper = 3,813 m/s• Rubber = 54 m/s

Tuning forks

• Tuning forks – touch to water in glass, touch to different materials how does that affect the sound/vibrations?

• The wavelength of the sound that the tuning fork emits is much larger than the width of the tuning fork’s tines. As a result, air has plenty of time to move around the tines during each cycle of vibration.

• Thus, instead of pushing air toward and away from your ear as it vibrates, each tine prevents the air from flowing around it and helps the tine push the air toward and away from your ear. You hear much more sound as a result.

Waves and Vibrations• Thunder Drum- The vibrating spring causes the drumhead to vibrate,

which in turn causes the air inside the drum to vibrate. The air in the drum begins to resonate, and the volume of the sound increases.

• Resonance – a phenomenon that occurs when two objects naturally vibrate at the same frequency.

• For example, when both the guitar body and the string are vibrating at the same frequency. The guitar body has a larger area than the string and is in contact with more molecules in the air. So the body is a better at transferring the vibrations to the air than the string is.

• How does this relate to the Thunder Drum?

Waves and Vibrations

• The natural frequency of an object depends on the object’s shape, size, mass, and the material from which the object is made

• Talking Cups

• Wine glasses with water – Your finger slides and sticks. Bowing the wineglass much the same way a violinist bows a violin string. With each cycle of vibration you add a bit of energy.

• Have you ever held a blade of grass between you thumbs and made it “sing”? How does cupping your hands around the blade help the sound?

Natural frequency• Singing Rod• Just like the wine glasses, guitar, singing cups, thunder drum the singing

rod has a natural frequency its particles want to vibrate at when energy is introduced.

• By holding the rod at the node (the place where the object is not moving) and vibrating the antinode the natural frequencies of vibration can be achieved.

• Touching a node will not dampen the sound; touching an antinode will.

Harmonic Vibrations

• Plastic swing tube

• How does the speed of the tube affect pitch?

• The air a the tube ends flows inward and outward together and the air at the middle of the tube experiences up and down pressure fluctuations, but no velocity fluctuations.

• The higher the tones the more places in the tube that are experiencing up and down pressure fluctuations.

Standing Waves• Carpet Tubes• What happens to the air in the tube as it is lowered over the flame?

• What happens to the pitch when the size of the tube changes?

• This is really the principle used by any wind instruments in a band or orchestra. The instrument has a hollow tube of some sort, its length will determine which frequencies it will amplify and its shape will determine the quality or type of sound it will give out. The musician produces a vibration, usually with the lips or a reed, and the air in the hollow tube will vibrate in resonance with whatever frequency it is designed to amplify.

Standing Wave

• Guitar or Violin String

• Pressing down on the string creates a node at that point.

• The location of the node determines the pitch of the note.

Interference• When several waves are in

the same location, they combine to produce a single, new wave that is different from the original wave.

• Constructive Interference increases the amplitude.

• Destructive interference decreases amplitude.

Gas Bottles with water and Tuning Fork• Each time the tuning fork is held above the glass bottles you are actually

hearing 2 sounds, not one.

• You are hearing the sound made by the tuning fork and the sound that reflects off the water and reemerges from the jar.

• When the reflected wave reemerges, it overlaps with the wave coming from the tuning fork.

• If the wave emerging from the jar overlaps constructively you will hear a loud sound.

• If the emerging wave overlaps destructively, you will hear no sound or a very low amplitude sound.

• How does this demo also relate to Resonance?

Doppler Effect• Motion between the source of

waves and the observer creates a change in observed frequency.

• Frequency changes the Pitch• Examples

– Ambulance– Doppler ball– Doppler Radar – Radar dome

receives radio waves that have been sent out an then reflected back by rain, snow, and hail. Meterorologist observe frequency shifts.

Do you remember learning about the Doppler Shift in space science?